The structure, function, and metamorphic reorganization of the abdominal stretch receptor organ in Manduca sexta.

Abstract

This dissertation examines whether proprioceptive feedback to the nervous system changes as an animal's morphology and behavior are modified postembryonically. The stretch receptor organ (SRO) in the abdominal segments of the hawkmoth, Manduca sexta, was investigated using a variety of neuroanatomical and electrophysiological techniques. The first goal was to describe the cellular composition of the SRO. A thorough description of SRO anatomy is essential to understand its role in the behavior of this insect. This description included the characterization of the central arborization pattern of the SRO sensory neuron (SRSN) and the identification of the SRO motoneuron. The results revealed that the SRO shares anatomical features with other muscle-associated proprioceptive organs while having a much simpler composition. The second goal was to determine whether the SRO undergoes postembryonic modifications during metamorphosis. These results revealed that the SRO is relatively stable throughout metamorphosis, suggesting that any postembryonic changes in proprioceptive input are not due to alterations in SRO morphology and enabling an investigation of the role of the SRO in behavior at any stage of the insect's life. The last goal was to determine the synaptic input from the SRO to motoneurons that innervate power muscles in the abdomen, and to evaluate whether postembryonic development modifies the proprioceptive circuitry. The results of these studies revealed that the SRO provides synaptic input to abdominal motoneurons and that SRO synaptic input to these motoneurons is reorganized as this insect undergoes dramatic changes in behavior. The SRO is therefore an excellent model for investigations of muscle-associated proprioceptive organs. Several experiments are proposed at the conclusion of this dissertation. These would extend the present study and emphasize the value of the SRO as a model for understanding proprioceptive function.